Re: [Question]: about 'cpuinfo_cur_freq' shown in sysfs when the CPU is in idle state

[Xiongfeng Wang]

Hi Ionela,

Thanks for your reply !

On 2020/6/10 17:40, Ionela Voinescu wrote:
> Hi guys,
> 
> Sorry for showing up late to the party, I was on holiday last week.
> 
> On Thursday 04 Jun 2020 at 13:58:22 (+0100), Sudeep Holla wrote:
>> On Thu, Jun 04, 2020 at 12:42:06PM +0200, Rafael J. Wysocki wrote:
>>> On Thu, Jun 4, 2020 at 6:41 AM Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
>>>>
>>>> On 04-06-20, 09:32, Xiongfeng Wang wrote:
>>>>> On 2020/6/3 21:39, Rafael J. Wysocki wrote:
>>>>>> The frequency value obtained by kicking the CPU out of idle
>>>>>> artificially is bogus, though.  You may as well return a random number
>>>>>> instead.
>>>>>
>>>>> Yes, it may return a randowm number as well.
>>>>>
>>>>>>
>>>>>> The frequency of a CPU in an idle state is in fact unknown in the case
>>>>>> at hand, so returning 0 looks like the cleanest option to me.
>>>>>
>>>>> I am not sure about how the user will use 'cpuinfo_cur_freq' in sysfs. If I
>>>>> return 0 when the CPU is idle, when I run a light load on the CPU, I will get a
>>>>> zero value for 'cpuinfo_cur_freq' when the CPU is idle. When the CPU is not
>>>>> idle, I will get a non-zero value. The user may feel odd about
>>>>> 'cpuinfo_cur_frreq' switching between a zero value and a non-zero value. They
>>>>> may hope it can return the frequency when the CPU execute instructions, namely
>>>>> in C0 state. I am not so sure about the user will look at 'cpuinfo_cur_freq'.
>>>>
>>>> This is what I was worried about as well. The interface to sysfs needs
>>>> to be robust. Returning frequency on some readings and 0 on others
>>>> doesn't look right to me as well. This will break scripts (I am not
>>>> sure if some scripts are there to look for these values) with the
>>>> randomness of values returned by it.
>>>
>>> The only thing the scripts need to do is to skip zeros (or anything
>>> less than the minimum hw frequency for that matter) coming from that
>>> attribute.
>>>
>>>> On reading values locally from the CPU, I thought about the case where
>>>> userspace can prevent a CPU going into idle just by reading its
>>>> frequency from sysfs (and so waste power), but the same can be done by
>>>> userspace to run arbitrary load on the CPUs.
>>>>
>>>> Can we do some sort of caching of the last frequency the CPU was
>>>> running at before going into idle ? Then we can just check if cpu is
>>>> idle and so return cached value.
>>>
>>> That is an option, but it looks like in this case the cpuinfo_cur_freq
>>> attribute should not be present at all, as per the documentation.
>>>
>>
>> +1 for dropping the attribute.
>>
> 
> I've been experimenting with some code quite recently that uses the
> scheduler frequency scale factor to compute this hardware current rate
> for CPPC.
> 
> On the scheduler tick, the scale factor is computed in
> arch_scale_freq_tick() to give an indication on delivered performance,
> using AMUs on arm64 [1] and APERF/MPERF on x86 [2]. Basically, this scale
> factor has the cached value of the average delivered performance between
> the last two scheduler ticks, on a capacity scale: 0-1024. All that would
> be needed is to convert from the scheduler frequency scale to the CPPC
> expected performance scale.
> 
> The gist of the code would be:
> 
> delivered_perf = topology_get_freq_scale(cpu);
> delivered_perf *= fb_ctrs.reference_perf;
> delivered_perf = div64_u64(delivered_perf << SCHED_CAPACITY_SHIFT,
> 			   per_cpu(arch_max_freq_scale, cpu));
> 
> While this solution is not perfect, it would provide the best view of
> the hardware "current" rate without the cost of waking up the CPU when
> idle, scheduling additional work on the CPU, doing checks on whether
> the CPU is idle and/or providing other caching mechanisms.

I think it's a good idea. It's just that the value is a average frequency in the
last two scheduler ticks, not an instantaneous frequency. What
'cppc_cpufreq_get_rate()' get is also not an  instantaneous frequency. It's a
average frequency in 2us. I check the interval between two frequency updates on
my machine. It's about 10ms. So the frequency will change at least one time in
two scheduler ticks if HZ is 1000.

I am not sure how people will use 'cpuinfo_cur_freq'. They may not expect a very
accurate frequency. How about we return this value when CPU is idle? Or just
return 0 in idle is better ?

> 
> Do you think such an implementation could make cpuinfo_cur_freq worth
> keeping?
> 
> I'm happy to push the patches for this and discuss the details there.

Thanks. I'm happy to see the patches and give some help.

Thanks,
Xiongfeng

> 
> Thanks,
> Ionela.
> 
> 
> [1] https://lkml.org/lkml/2020/3/5/183
> [2] https://lkml.org/lkml/2020/1/22/1039
> 
>> -- 
>> Regards,
>> Sudeep
> 
> .
>